This proposal will examine monocyte heterogeneity during infectious diseases. We have identified subsets of murine monocytes that behave very differently during infection with the protozoan parasite, Leishmania major. One monocyte subset is rapidly mobilized from the blood in response to L. major infection, and the mechanism of monocyte migration appears to be unique to this subset of monocytes. We have termed these rapidly migrating cells effector monocytes because they are remarkably efficient at killing L. major parasites. We will examine human monocyte subsets and determine whether a subset of human monocytes are also efficient killers of L. major and other intracellular pathogens. We propose to examine the mechanism(s) of monocyte killing, and examine the role of the monocyte respiratory burst (NOX2) in parasite killing. In contrast to the effector monocytes, the so-called classical monocyte population fails to efficiently kill L. major. We propose to compare and contrast these two subpopulations of murine monocytes, and then determine whether human monocyte subsets are similarly susceptible to intracellular infections. In aim 3, we propose to utilize next generation RNA sequencing to compare the transcriptomes of the two murine and three human monocyte subsets during infection with L. major. These studies should provide insight into the nature of the monocyte responses to infection, and how transcriptional changes in the parasite are correlated with parasite intracellular survival. This work will provide the first comparison of the major monocyte subsets and their responses to parasite invasion.